This invention relates generally to gate valves and more particularly to an improved valve seat structure for expanding gate valves.
Gate valves of the expanding gate type typically employ a gate mechanism which is mounted in a valve chamber and is movable therein transversely of the flow passage to open and close the valve. The gate assembly typically comprises a gate member and an adjacent segment member which, as the gate assembly approaches the open and closed positions, are expanded transversely of one another by cooperating cam surfaces whereby they are forced against seat rings in the valve body on opposite sides of the gate assembly for effecting upstream and downstream seals. The gate assembly includes parallel sealing surfaces which are maintained parallel as the gate assembly expands in the open and closed positions to seal against the seats. Upon movement from the open and closed positions, the gate assembly collapses from its expanded condition to permit reciprocal movement of the gate assembly without excessive friction between the gate assembly and the seat assemblies.
Most generally, the seat assemblies for expanding gate valves are retained within seat recesses which are formed within the valve body in surrounding relationship to the flow passage on each side of the gate assembly. The valve seats are in the form of seat rings which in one widely used valve structure are pressed into the seat recesses to that the seat will be retained in a fixed position. Ideally, the seats provide essentially planar and parallel surfaces for the gate assembly to contact and seal agains where it is in the expanded configuration regardless of whether the gate assembly is in the open position or the closed position. As a practical matter, however, the maintenance of parallelism between the outer sides of the gate assembly and the faces of the valve seats is frequently a problem which impairs the ability of the valve to achieve a seal. In addition, the seats do not always remain fixed and they may float or move inwardly toward the gate assembly under high differential pressure conditions. In doing so, the seats move to a position tight against the gate assembly and drag excessively thereon thus making the valve very difficult to open or close. An alternative construction to the fixed press-fit seat ring is the floating seat arrangement wherein the seat member is designed to float in the seat pocket in the direction of the flow passageway so that it can be moved against the gate sides by the fluid pressure. Such floating seats have the advantage of being able to seal against the gate and segment members even if there is a lack of parallelism or if there are irregularities in the mating surfaces of the gate assembly and the valve seat. A potential disadvantage of this type design is that excessive drag can be created between the seat members and the gate assembly when the gate assembly is moved due to the upstream seat being forced against the gate assembly by fluid pressure.
Furthermore, under high pressure service conditions in excess of 10,000 psi, there must be a smaller clearance between the seat ring and seat pocket to preclude damaging of the gate sealing surface by the misalignment of the metal seat ring. Accordingly, a reduced ability of such a floating seat to compensate for non-parallelism is associated with the smaller clearances. In addition, the operating torques for expanding the gate assembly to achieve a seal under high pressure service conditions become extremely high so as to make valve operation very difficult.
It is therefore an object of the invention to provide a gate valve of the expanding gate type with a unique improved "floating" valve seat construction wherein the seat element tends to be retained by differential flowline pressure in the open and closed conditions of the valve and as the gate assembly begins its movement to its collapsed condition intermediate its open and closed positions and is able to compensate for non-parallelism between the cooperating sealing surfaces of the valve seat and gate assembly, particularly in high pressure service conditions.
Another object is to provide a gate valve with an expansible gate assembly and valve seat means comprising a "floating" seat ring adapted to be urged by flowline pressure towards the back of the seat pocket to eliminate "drag" between the gate assembly and the valve seat as the gate assembly moves between open and closed positions.
A further object is to provide an expanding gate valve with "floating " seat rings, each with annular grooves with sealing elements in the front and rear faces and each sealing element having pairs of spaced frusto-conical surfaces adjacent its rear face for cooperating with conforming wedge surfaces in said grooves whereby the sealing elements in the upstream seat are cammed away from the flow passage to effect a seal and those in the downstream seat are cammed towards the flow passage to effect a seal in the event of leakage of the upstream seal.